1. Field of the Invention
This invention relates to artificial joints and particularly to modular proximal femoral hip prostheses, and a method and instrumentation for implantation of the same.
2. Description of the Prior Art
Various prostheses have heretofore been designed to replace one or both components of a ball and socket hip joint. Generally the ball portion is connected to an arm composed of a neck and a stem or shaft which stem or shaft is embedded in the intramedullary canal of the proximal femur for hip reconstruction. Such prostheses are often formed with an integral stem and neck portion. Often a removable ball or head element is positioned on the proximal end of the neck. See, for example, U.S. Pat. Nos. 4,012,795 or 4,459,708.
Recently the use of modular structures fitted together from a number of replaceable parts available in a variety of sizes have been used. With such prostheses it is possible to replace either the head portion or trochanteral portion of the prostheses, or both, without removal of the stem from the bone cavity. U.S. Pat. Nos. 4,608,055, 4,676,797, and 4,693,724 are all illustrative of such devices. The latter patent also discloses the possibility that the angle at which the neck protrudes from the proximal end of the femur (referred to in said patent as "anteversion") may be adjusted without removal of the stem by pivoting the neck on the end of the implanted stem. None of the prior art devices of which applicant is aware provide any means for varying the angle between the axis of the trochanteral module and the axis of the stem so that the actual angulation (sometimes referred to as anteversion) or slope of the proximal end of the femur may be duplicated by adjustment of said angle.
The present invention provides a modular hip prosthesis and instrumentation for implanting the same, which has provision for varying the angulation between the stem portion and the trochanteral, or neck, portion by provision of connection means between the trochanteral module and stem which can be positioned or attached together in a variety of rotational positions. Variation of the angulation or anteversion is made possible in accordance with the present invention by virtue of the fact that the axis of the connection portion between the stem and trochanteral portion is angularly offset from the axis of the body of the stem. The prostheses of this invention are further characterized by the fact that no impediment exists to securing the stem and neck together in a variety of rotational positions. Like other modular prostheses the hip joint prostheses of this invention provide the advantage that either or both the ball component or trochanteral module component can be removed if replacement becomes necessary without extraction of the stem from the bone canal. Different size balls or trochanteral components could also be substituted should the surgeon decide that such revision is necessary after a period of time.
A further aspect of this invention is the provision of instrumentation for formation of a cavity for implantation of a prosthesis of this invention which is provided with indicia or markings for indicating to the surgeon the optimum angle for assembly of the prosthesis of invention for implantation into a particular proximal femur. The invention further provides a novel method for determining the proper angulation for a particular proximal femur using angulation readings taken off of indicia provided on the rasp used to prepare the proximal femur for the prosthesis. Such readings then provide a basis for assembly of the prosthesis in the correct orientation to fit the proximal femur.
Briefly summarized, the present invention provides a modular hip joint prosthesis having provision for varying the angulation of the stem portion and the trochanteral module to coincide with the degree of angulation in the proximal femur of a patient. The prosthesis includes a ball, a stem component for implantation in a proximal femur, a trochanteral module adapted to be connected at its proximal end to said ball and at its distal end to said stem component by a connection portion. The connection portions of the stem and trochanteral components have no mechanical impediment to limit securing the two components together in a variety of rotational positions. The trochanteral module component is of an oblong cross-section on a plane perpendicular to the longitudinal axis of its distal connection portion. It, further, is symmetrical in shape across the plane in which the axis of the ball connection shaft is located, but is preferably non-symmetric in all other planes. Thus when the stem component is rotated in different orientations relative to the trochanteral module, a tilting of the trochanteral module occurs relative to the stem, making it possible to closely match the natural tilt of the proximal femur. The stem component is provided with a connection portion which, as already noted, mates with the connection portion of the trochanteral module in a variety of rotational positions, with the axis of the connection portion of the stem being angularly offset from the axis of the body of the stem. The implant is provided with markings or indicia positioned circumferentially around the connection portion so that the surgeon can match the orientation of the implant with that of the instrumentation as observed from the indicia thereon when the rasp portion of the instrumentation is imbedded in the proximal femur.
The novel surgical instruments of this invention include a stem component which has a connection portion offset at an angle identical to the angle at which the connection portion of the implant stem is offset from the axis of the implant stem. The instrument connection portion is adapted to receive a series of rasp block portions of progressively increasing dimension. Each of the rasp block portions are of a shape corresponding to the trochanteral modules of the implants. The rasp blocks and the instrument connection portion each have markings thereon located circumferentially around the connection portion for determination of the relative orientation between the rasp block and the stem axis. The neck of the implant and connection portion of the implant stem have similar markings to allow the surgeon to assemble the implant in the same orientation as observed on the rasp instruments after they have been inserted in a manner and orientation deemed to be optimal by the surgeon.
The novel method of the invention is as follows: the surgeon begins by removing the head of the proximal femur this resection being a rough resection. The surgeon then inserts a starter reamer into the medullary canal of the femur and sequentially reams using reamers of progressively increasing diameters until he finds a reamer of the appropriate diameter which contacts the cortical bone. The reamers employed are cylindrical in shape and have helical or longitudinal flutes which cut the bone. Once the appropriate diameter for the distal stem is thus determined by means of the reaming, the surgeon selects that same diameter distal trial stem and assembles onto it a proximal rasp portion which mimics the geometry of the trochanter module of the actual implant. The distal trial stem also corresponds to the geometry of the distal stem of the implant. The proximal rasp has a hole through it that has a center line that is coincident in orientation to the center line of the connection portion on the actual proximal wedge of the implant. The trial stem has a long cylindrical stud that projects proximally off of the distal stem at an angle that is the same angle as the connection portion on the actual distal stem, but this stud is cylindrical and has a length sufficient such that when assembled through the hole in the proximal rasp, the stud extends out of the other side of the proximal rasp and is then attached to the driving handle. The driving handle holds the entire assembly together by engaging the end of the stud on the trial stem by means of an interlock mechanism. The entire assembly is locked together such that the proximal rasp can rotate on the distal stem but is axially retained on the driving handle. The distal stem can freely rotate about the center line of the stud axis, and the handle and proximal rasp may be locked together so that they rotate as one unit. Since the surgeon has already selected the distal trial stem by reaming, the only component that is changed during the sequential rasping is the proximal rasp. The surgeon begins with a small proximal rasp and progressively utilizes larger rasps until he determines the size which will best fit the proximal region of the bone. During the rasping process the surgeon is free to rotate the proximal rasp relative to the distal stem and thus determine an optimum orientation of the rasp. He will visualize the best position for the proximal rasp each time he drives the proximal rasp down into the bone. When he finally reaches the appropriate size rasp he will leave the rasp and distal stem in the bone but detach the driving handle so that he can see the top surface of the rasp and the stud projecting out through the hole in the rasp. The stud is provided with a line or other mark and the top surface of each rasp adjacent the opening is provided with a series of lines or marks. Each of these lines will be at approximately 111/2 (or some other selected interval) apart from each other to indicate to the surgeon the orientation of the rasp on the rasp stem. The implant trochanteral module and implant stem are provided with identical markings so that the surgeon is able to assemble them in an orientation identical to that which was observed between the rasp and rasp stem. Hence the amount of anteversion in the actual implant when it is assembled is the same as that which was determined by virtue of the rasping procedure. The lines on both the rasp and implant neck are marked with letters or numbers, for example, 1 through 9 or A through J, etc. The surgeon thus can observe the designated orientation of the rasp and distal trial stem and assemble the prosthesis using the markings as a guide. The surgeon will thus assemble the distal stem onto the proximal wedge in the same position that was read off of the instrumentation, thereby insuring that the amount of angulation in the femoral implant is the same as the amount of angulation in the bone.